When we looked at the Amish who were the most active, there is suddenly no effect of that gene," said Dr. Soren Snitker of the University of Maryland, whose study appears in the Archives of Internal Medicine.
The findings, which suggest physical activity can overcome a genetic predisposition for obesity, may help inform the debate over whether changes in diet or physical activity will make the biggest difference in fighting obesity.
Consumer groups have pushed for laws such as July's moratorium on new fast-food restaurants in certain Los Angeles neighborhoods, while the food industry often maintains that a lack of exercise is more to blame.
Researchers focused their study on a group of 704 Old Order Amish men and women in Lancaster County, Pennsylvania, a religious group whose members often do not drive cars or have electricity in their homes.
Snitker said the group offered a unique mix of activity levels, with some farmers in the community still using horse-drawn plows while others holding more conventional jobs, including factory work.